How to Make It
Preheat oven to 375°F. Coat a 6-cup, preferably oval, baking pan or dish with cooking spray.
Combine rhubarb, apples, granulated sugar, 2 tablespoons flour, vanilla extract, and cinnamon in a medium bowl; toss well, and place in prepared baking dish.
Combine remaining 6 tablespoons flour, oats, and brown sugar in a bowl; mix well. Add butter to mixture; rub in with your hands until the mixture begins to hold in clumps when compressed. Sprinkle oat mixture evenly over rhubarb mixture in baking dish.
Bake at 375°F until topping is golden brown, filling is thick and bubbly, and fruit is tender, about 50 to 55 minutes. (Cover loosely with foil if the top begins to brown too quickly.) Cool 15 minutes before serving.